[Imp]

Bunkers are not like vehicles with regard to armour even small arms fire can penetrate and kill those manning it if your lucky.

Not looked at your save but continuous close fire at a bunker normally creates a critical hit pretty quickly.

An experienced crew firing stationary from 1 or 2 hexes have a high probability of a quick kill. Same goes for high experience troops armed with demo or flame so long as you wait for them to be stationary before attacking.